Climate Change Impacts on Four Agricultural, Headwater Watersheds from Varying Climatic Regions of New Zealand

Abstract

Climate change impacts on air temperature, precipitation and surface flows in four agricultural, headwater watersheds (Toenepi and Waiokura in North Island and Inchbonnie and Bog Burn in South Island) from varying climatic regions—warm subtropical to cool temperate—of New Zealand were examined using a suite of soil moisture accounting models. The models were calibrated and validated using observed flows from the four watersheds. Using the validated models and published climate change data from two future periods, 2030–49 (referred to as 2040) and 2080–99 (2090), and one current period, (1980–99, referred to as 1990), the impacts on air temperature and hydrology were investigated. The future periods considered three emission scenarios: low (B1), medium (A1B) and high (A1FI). The model outputs were analysed for changes in air temperature, precipitation and surface flows from 1990 conditions. Under climate change, temperature, precipitation and surface flows were predicted to increase from low to high emission scenarios, and from 1990 conditions. As the climate warms up, the warm subtropical Toenepi watershed was predicted to undergo less warming than the cool temperate Bog Burn watershed, though the former would undergo more frequent dry spells (days without rain) and low flows than the latter. With climate change, agriculturally, non-growing season (April–September) is likely to record greater temperature increases and more intense precipitation than growing (October–March) season. In all four watersheds, climate change was predicted to result in increases in mean and maximum daily precipitation. Soil moisture accounting models predicted that this could result in increases in maximum daily flows in the two South Island watersheds. However, owing to extended dry spells, the two North Island watersheds are likely to record decreases in mean and maximum flows. As all the four watersheds considered are dominantly animal grazed, reduction in precipitation during the growing season may negatively impact pasture growth. To sustain agricultural productivity, the farmers and producers may need to explore other measures such as irrigation, reduced stock density, importation of feed, use to drought-resistant pasture species and/or development of water storage options to counter dry spells, all of which may result in significant to transformative changes to agricultural practices and farmer water use behaviour.